The present invention relates to a system for driving elements of an electrochromic (hereinafter abbreviated to ECD) display cell, and more specifically, to a system for driving elements of an ECD cell whereby a single cell element can attain a plurality of stable coloration density states, so that such an element can perform a plurality of functions.
At the present time, liquid crystal display cells are widely utilized in various types of electrical equipment, and particularly in portable electronic devices such as electronic timepieces. With such a liquid crystal display, each display element can attain only two display states, e.g. clear state and a dark state. It is a characteristic of liquid crystal display cells in general that the degree of cell contrast varies in dependence on the angle from which the cell face is viewed. Thus, even if it were possible to produce a liquid crystal display cell having three or more display states, such a device would not be practical, due to the changes in display contrast which result from changes in the viewing angle. With an ECD cell, however, the degree of display contrast does not vary with the viewing angle. In addition, it is possible to establish a plurality of display density states for the elements of an ECD cell, e.g. a colorless state, a dark or densely colored state, and one or more states of intermediate color density. Various proposals have been put forward in the prior art whereby these properties of ECD cells are used to provide displays in which a display segment performs two different functions by attaining two different coloration density states. However such proposals have been vague and nonspecific, and no practical system for implementing an ECD cell drive system has been disclosed which would not be extremely complex and which would meet the most important requirements for such a system. These requirement will be briefly described, referring to Table 1 below.
TABLE 1 ______________________________________ COMBINATIONS OF SEGMENT DISPLAY STATE CHANGES ______________________________________ 1 clear - clear 6 grey - dark 2 clear - grey 7 dark - clear 3 clear - dark 8 dark - grey 4 dark - clear 9 dark - dark 5 grey - grey ______________________________________
Entries 1 to 8 in Table 1 denote each of the various combinations of changes in display state which can occur for a segment of a CMOS cell. Thus for example, entry 2 denotes the change from the clear display state to the grey display state. Entry 8 again indicates a change by which the grey state is attained, but in this case a transition is made from the dark state into the grey state. It is an essential requirement for a satisfactory drive system to provide such a plurality of display states that the color density of the grey display state resulting from a change from the dark level must be identical to the density of a grey state which results from a change from the clear state. Similarly, it must be ensured that the density of a dark display state which results from a transition from the grey state is identical to the density of the dark state which results from a change from the clear state. Unless these requirements are met, it will not be possible to provide a satisfactory ECD display device in which display segments can attain a plurality of coloration density states. No system has been disclosed in the prior art which will meet these requirements and which is at the same time sufficiently free from complexity to be suitable for practical realization. However such a system is disclosed by the present invention, as will be made clear in the specification.
It will be noted that for certain entries in Table 1 above, no actual change in segment display density occur, e.g. as in the case of entries 1, 5 and 9. These correspond to a condition in which, when a periodically performed check is carried out to determine whether a change in display state has been designated, it is found that no change is required, and the segment is therefore left in the same display state. This can be generally achieved, with an ECD cell, by leaving the segment in an open-circuit condition so that no charge is discharged therefrom.